It has been found necessary to transform the harmful materials contained in the exhaust gas of combustion engines, such as carbon monoxide (CO), hydrocarbons (C.sub.x H.sub.y) and nitrogen oxides (NO.sub.x) by suitable measures into harmless chemical compounds. For this purpose, thermically and catalytically acting reactors or afterburners are being used.
When a combustion engine is operated at varying power, the composition of the exhaust gases varies continuously. This disturbs the afterburning, because either too much or too little oxygen is available for afterburning. It is already known in the art how to avoid this disadvantage by operating the combustion engine with a deficiency of air (.lambda.&lt;1) and to deliver the combustion air necessary for afterburning through a motor-operated air pump. Such a device is shown and described in the German Disclosure Document No. 2,035,591.
According to the German Disclosure Documents Nos. 2,012,118 and 2,120,950, the metering of the supplementary air is accomplished by switching devices to which several command variables can be provided as input. According to the German Disclosure Document No. 2,064,266, there is known a switching device for exhaust gas return which also may be influenced by several command variables.
However, with the known devices, the switching conditions vary discontinuously and therefore make continuous regulation impossible.
Another disadvantage of the known devices is that the amount of supplementary air is not controlled discretely and with sufficient speed or reaction time. Hence the afterburner cannot operate at an optimum and the harmful materials are not transformed or converted with sufficient effectiveness.
According to U.S. Pat. No. 3,931,710 it has already been proposed that several command variables be applied to the regulating device from the outside. At the same time, the quantity of air entering the regulating device is metered as a function of the engine speed by means of an air pump driven by the engine; the regulating device comprises a device for the removal and return of part of the air entering the regulating device as a function of the command variables.
In the operation of such a regulating device, trouble may occur when too many unburned hydrocarbons, via the exhaust manifold, reach the afterburner and are combusted there in excessive quantities. Especially with catalytic afterburners, the afterburner may be ruined in a very short time.
It is, therefore, an object of the present invention to remedy this disadvantage and to prevent destruction of the afterburner as a result of improper operation.
Another object of the present invention is to provide an arrangement of the foregoing character which is simple in design and construction, and may be economically fabricated.
A further object of the present invention is to provide an arrangement, as described, which has a long operating life and which has part readily accessible for maintenance and service.